1. Technical Field
The present invention relates to a control system and method for changing speed in starting an engine of a hybrid vehicle, and more particularly to a control system and method that is further capable of preventing the occurrence of shift shock.
2. Background Art
In general, hybrid vehicles which adopt an engine as well as a motor driving source as an auxiliary power are future oriented vehicles capable of reducing exhausts and improving fuel efficiency.
With reference to FIG. 1, the constitution of a power train for transmitting power of such a hybrid vehicle is described below as an example.
A power train for the hybrid vehicle in FIG. 1, comprises an engine 10, a first motor MG1 controlling a change in speed of an engine crank and an engine, a second motor MG2 that is a traction motor for directly transmitting torque to the axle, and a pair of planet gear sets 20, 22.
The output axle of the engine 10 is connected with a carrier C1 of the first planet gear set 20, and at the same time is connected with a second sun gear 52 of the second planet gear set 22 through a second dutch CL2.
The output axle of the first motor MG1 is directly connected with a sun gear S1 of the first planet gear set 20, and the output axle of the second motor MG2 is directly connected with the second sun gear S2 of the second planet gear set 22.
As shown, a ring gear R1 of the first planet gear set 20 is connected with a carrier C2 of the second planet gear set 22, which in turn is connected to the final output axle.
Also, a ring gear R2 of the second planet gear set 22 is connected to the output side of the carrier C1 in the first planet gear set 20 through a first dutch CL1.
As further shown, a first brake BK1 is equipped in the connecting axle between the first motor MG1 and the sun gear S1 of the first planet gear set 20, and a second brake BK2 is equipped in the connecting axle between the ring gear R2 of the second planet gear set 22 and the output side of the carrier C1 in the first planet gear set 20.
In the power train structure of such a hybrid vehicle, torque for cranking the engine is generated in the first motor MG1 which controls the engine speed. The second motor MG2 performs the traction motor function directly transmitting torque to the axle, and at the same time, controls the speed of the ring gear R2 of the second planet gear set 22 so as to follow the target speed based on the driving situation.
In the power train system of the hybrid vehicle described above, all the functional elements including the brake and the dutch are enabled and disabled by oil-pressure control under the neutral position (P or N) of the movable running gear.
For example, for shifting to the driving gear (i.e. R or D gear) from the neutral gear i.e. P or N gear) with ease, it may be controlled such that the speed of the ring gear R2 in the second planet gear set 22 is maintained at 0 so as to connect with the second brake (Brake 2) easily. Alternatively, it may be controlled such that the speed of the ring gear R2 in the second planet gear set 22 is the same as that of the carrier C1 in the first planet gear set 20 for easily connecting with the first dutch (Clutch 1).
Accordingly, since the speed of the ring gear R2 in the second planet gear set 22 maintains the target speed during general driving situations (after starting the engine), a shock state is not generated under the static shift to the D gear from the N gear, or to the N gear from the D gear. However, a shock state may be generated when the static shift is performed during engine start-up.
Namely, in order to follow the target speed of the ring gear R2, a feedback control using, for example, a PI control part (i.e. proportional-integral control part) may be preformed against the speed of the second motor MG2. However, the target speed of the ring gear R2 is not maintained due to the falling of the transient response (the response as the output goes to the normal state) in controlling the feedback. This generates a shock state when the functional elements of the second planet gear set are connected.
As such, once the static shift to the D or R gear from the P or N gear is performed during engine start-up, the reacting power corresponding to the engine friction works toward the output axle of the second motor MG2, which generates an instant drop in speed of the second motor MG2. As a result, the ring gear R2 in the second planet gear set 22 deviates from the target speed instantly to generate a shock state when shifting from the neutral gear (P or N) to the driving gear (R or D).